1. Field of the Invention
The invention relates to a method of controlling a satellite communication system which comprises satellites and ground network system devices positioned on earth. The invention also relates to a timing unit and a control unit of a timing unit.
2. Description of the Related Art
Positioning a subscriber terminal, i.e. determining its geographical position, has become an increasingly important function in cellular radio networks. In the United States, for instance, the Federal Communication Commission (FCC) requires that all subscriber terminals calling an emergency call must be positioned even at an accuracy of 50 meters. Positioning applications also include navigation, flight control prediction of continental displacement and meteorology.
In time-synchronized networks, the subscriber terminal could measure the reception time interrelations of the signals received from different base transceiver stations, on the basis of which interrelations the location of the subscriber terminal could be defined. In practice, however, radio systems are not completely time-synchronized, which would cause errors in positioning. This problem has been solved in some positioning methods according to the prior art by bringing into use a location measurement unit (LMU) positioned at a fixed, known measurement point. Such methods include E-OTD (Enhanced Observed Time Difference) and IPDL-TDOA (Idle Period DownLink—Observed Time Difference Of Arrival) methods. By means of the LMU, the real time difference between the transmissions of base transceiver stations is determined. The effect of real time differences is eliminated from the results measured by the subscriber terminal, whereby the location of the subscriber terminal can be determined. The location of the subscriber terminal is obtained from the intersecting point of the hyperbolas expressing the propagation time delays in the coordinates of the base transceiver stations.
In several positioning methods, such as in the E-OTD method, the time difference between the base transceiver station clocks is most frequently determined by using the real time difference (RTD) between the base transceiver stations, which is determined by means of the LMU on the basis of the signals received from the base transceiver stations. A problem with the determination of the time differences between the base transceiver stations based on RTD determinations is, however, that it requires a large amount of calculating capacity. In order to reduce the calculating capacity, what is called the use of absolute time (AT) has been provided.
In the GPS system (GPS, Global Positioning System), a receiver receives a signal transmitted by at least four satellites revolving around the earth, on the basis of which signals it becomes possible to calculate the latitude, longitude and altitude of the subscriber terminal location. All in all, the system contains 24 satellites revolving around the earth. In practice, however, it is often the case that fewer satellites are visible, in which case the accuracy of the positioning is reduced. The visibility of the satellites can be poor in the shadow regions of high buildings and in the vicinity of the poles, for example, because the visibility angles of the satellites are usually small. In the positioning missing satellites can be compensated for by using UTC time (Coordinated Universal Time).
Thus, the calculation of time differences related to the synchronization and positioning of a radio communication system is in many prior art methods based on using absolute time or UTC time (Coordinated Universal Time). However, a problem with the use of absolute time or UTC time is that for different reasons, synchronization errors are caused in the absolute time or UTC time. In systems where a signal is received from a satellite, an error is caused for instance by a radio signal being distorted in the atmosphere. In positioning methods, even a small timing error can cause significant inaccuracy; for instance a timing error of 100 nanoseconds means inaccuracy of about 30 meters in the positioning.
In addition, it can be mentioned as a problem with the GPS system that a GPS receiver is needed for the subscriber terminal, which means, for instance, that the size, weight and costs of the subscriber terminal are increased.